Switching Power Supply Switch
Switching power supply switch tube from the conduction to the cut-off, strictly speaking, is a very complicated process, but when we work principle analysis, will first try some non-major problem simplistic. For example, when the power switch tube turned on or off, we put it as an ideal switch its work only two states, on or off. But fact the switch tube conduction and off is a very complicated process, which in addition to ON or OFF, there is a high frequency can not be neglected, that is, the switch conduction is from the cutoff region to the enlarged area, and then by the enlarged area to work in saturation region. This process requires differential equations can be solved here, I do not want to introduce to you, too complicated.
Simply put, the power switch turn-on and turn-off time are needed. Generally simply switch conduction time ton into the turn-on delay time td and conduction rise time tr and switch off time toff to divided into close delay time tstg (or close the storage time) and shut down the decline time tf.
Figure 1 is a power switch turned on and off during the test waveform in fact the test waveform in Figure 1 is also simplified the actual waveform, the rising edge of the waveform is generally a rate of change of the positive index curve, while The falling edge of the waveform is a rate of change is a negative exponential curve, for simplicity, in Figure 1 are replaced with a straight line.
Switching power supply in the first cycle, the output voltage of the the filtering energy storage capacitor charge because the charging current is large, the load will be heavy (or equivalent load short-circuit), so the switching power supply in general should take a soft start measures the beginning of the duty cycle is very small, before slowly become normal, that began when the output power is very small, then slowly larger. Or the beginning of time, low operating voltage, then slowly increased to normal.
Strictly speaking, the switching power supply is always working in the non-steady-state, the so-called only relatively stable. For example, the process of switching power supply regulators is this: When the output voltage increases, after sampling, the sampling circuit outputs an error signal to a pulse width modulation circuit, so that the duty cycle is reduced, so that the output voltage is reduced; when After the output voltage is reduced, compared After sampling, the sampling circuit will output an error signal to a pulse width modulation circuit, the duty ratio is increased, so that the output voltage increases, so that repeated cycle, the output voltage of the switching power supply will always be at a certain the frequency of the average value of the voltage up and down swinging, but the average of the output voltage relatively stable as that so-called regulator.
For the current flowing through the the switching transformer primary coil is not a stable value, are generally a sawtooth wave rectifier output current. LED constant current drive , is generally means after filtering by the filter output current is relatively stable, and this stability is also means average, while the input current of the filter are generally a sawtooth wave.
Switching power supply start cycle is generally considered to begin from the switch conduction operator, which mainly depends on where you want to analyze the circuit cut refers switching power supply circuit when work, this can be considered If you need to analyze the various points of the waveform, you must take a device in the circuit waveform as a reference point (or sync) from the power switch is turned on to start work.
Usually sampling at the beginning of the first cycle of the switching power supply circuit is basically not working, because the output voltage on the filter capacitor is charged, the need can be charged to the normal after several cycles, only the output voltage reaches the normal, the sampling circuit to work properly. However, the sampling circuit is not working properly before the absolute value of its output voltage is equal to 0, which can be considered a special case of the error signal output (negative maximum). In such cases, switching power supply does not have a soft-start circuit, when the switch work, the duty cycle will easily transformer saturation, and damage to the switch.
